1. Field of the Invention
This invention relates to an improvement of metal cords used as a reinforcing member in rubber articles, particularly a belt of a pneumatic tire which can simultaneously establish resistance to cord breaking and weight-saving. Further, it relates to pneumatic tires having improved rolling resistance and service durability by using such metal cords.
2. Related Art Statement
In conventional rubber articles, particularly pneumatic tires, metal cords are widely used as a reinforcing member. For instance, the metal cord of a so-called 1.times.4 or 1.times.5 structure obtained by twisting 4 or 5 metallic filaments is used as a reinforcing member in a belt of a radial tire for a passenger car.
In using the metal cord as a reinforcing member, it is important that the metal cords are properly embedded in a coating rubber and sufficiently bonded thereto. If the coating rubber does not sufficiently penetrate into the inside of the metal cord or the bonding state between the metal cord and the coating rubber is poor, the metal filaments constituting the metal cord are corroded by water contained in the coating rubber or penetrated from the outside and the like to undesirably cause the breaking of the metal filament or separation failure between cord and rubber.
In order to solve these problems, there have hitherto been examined various methods. In U.S. Pat. No. 4,399,853 is disclosed a metal cord having such an irregular section distribution in longitudinal direction inclusive of separation regions between mutual filaments at an inequal interval as well as local contact regions separated between at least one adjoining mutual filaments but contacted between the remaining mutual filaments. In U.S. Pat. No. 4,333,306 is disclosed a steel cord having such a twisting structure that the loose twisted portion and the compact twisted portion are alternately arranged with each other in the longitudinal direction of the cord.
On the other hand, the weight-saving of the pneumatic tire is demanded from viewpoints of the improvement of rolling resistance in the pneumatic tire and resource-saving. In this connection, it has been attempted to simplify the twisting structure of the metal cord used as a reinforcement for thinning the thickness of the reinforcing layer in the pneumatic tire. For instance, U.S. Pat. No. 4,464,892 discloses a metal cord comprised of three metal filaments as a reinforcement in an elastic article such as pneumatic tire or the like, wherein the two metal filaments are twisted in a given direction to form a strand and the remaining metal filament is spirally arranged around the strand in a direction opposite to the twisting direction of the strand.
In any case, however, the improvement of the metal cord is attempted from only a viewpoint of the cord structure, and it is not attempted to synthetically solve the problems of the filament breaking in the metal cord and the separation between cord and rubber from viewpoints of the structure and shape of the reinforcing belt layer.
In the metal cord disclosed in U.S. Pat. No. 4,464,892, it is intended to penetrate the coating rubber between the strand composed of two metal filaments and the twisted metal filament. In this case, however, the twisting operation should be repeated two times, so that the production cost becomes expensive and also the compressive fatigue properties of the cord is poor.
When the twisted filament number is three, if the twisting is carried out so as to alternately arrange the loose twisted portion and the compact twisted portion per cycle as described in U.S. Pat. No. 4,333,306, the loose twisted portion is compacted under a tension, so that when the cord is used as a reinforcement for the pneumatic tire, it is difficult to penetrate the coating rubber into the interior of the cord.
In order to overcome the aforementioned problems and provide a metal cord with a twisting number of 3 facilitating the penetration of the coating rubber thereinto, the metal cord disclosed in U.S. Pat. No. 4,399,853 is considered to effectively change so as to further facilitate the penetration of the coating rubber.
Further, in case of enhancing the resistance to cord breaking and the resistance to separation between cord and rubber in the cord reinforcing layer of the belt, it is demanded that not only the metal cord is improved, but also the improved metal cord is advantageously applied to the pneumatic tire. However, such a demand is not yet satisfied.